The present invention relates to antenna systems for satellite radio communications, and more particularly, to a passive coupling device for a satellite radio antenna system.
Until relatively recently, satellite-based communication systems were used mainly for the transmission of telephone conversations and television broadcasts. Now satellite-based communication systems are being used to transmit radio broadcasts. In particular, the radio industry has recognized that satellite transmission of radio broadcasts allows listeners in cars, trucks, boats, and other vehicles to receive desired radio programming beyond the relatively limited geographic range associated with standard AM and FM radio broadcasting. Thus, for example, using satellite systems a listener can listen to the same radio station across an area of thousands of miles. An example of one currently available satellite radio broadcast service is the Satellite Digital Audio Radio Service (“SDARS”).
In order to receive satellite broadcasts, vehicles must be equipped with proper antennas and receivers. Since most vehicles are not yet built with such antennas and receivers as standard equipment, satellite-capable antennas and receivers must be retrofitted on and in the vehicles. Mounting appropriate antennas on existing vehicles presents a particular challenge since it is preferred that the antenna be mounted on the exterior of the vehicle and the receiver be mounted in the interior of the vehicle. Of course, it is also preferred that a wired connection be made between the antenna and receiver.
In many retrofitting applications, glass-mounted antennas are used because of their easy installation. Installing a glass-mounted antenna does not require drilling holes in an exterior vehicle surface in order to mount the antenna and to connect a wire or cable between the antenna and receiver. Thus, a glass-mounted antenna avoids air and water leakage problems, and allows the antenna to be removed from the vehicle without sealing or repairing holes. Although temporarily installed magnet-mounted antennas are available, they are visually obtrusive and require the cable to be passed through an existing door or window opening. As a result, the cables are often damaged.
While glass-mounted radio frequency (“RF”) coupling devices avoid the problems of conventional antennas, they introduce different concerns. Current glass-mounted RF coupling devices used in terrestrial cellular communication (which operate in the 800 and 1900 MHz frequency range) exhibit insertion loss characteristics of about 1½ to 2 dB. When these devices are used in a satellite radio transmission system (particularly those that operate above 1 GHz), the loss characteristics increase to an unacceptable level. Loss characteristics are not acceptable due to an increase in the system noise figure (“NF”) from the coupler.
Some glass-mounted RF coupling devices compensate for their loss characteristics by using an externally-mounted, low-noise amplifier (“LNA”) or other electronics to boost the received signal. While this arrangement may produce more acceptable characteristics, the externally mounted electronics are subjected to environmental hazards and possible tampering. An externally mounted LNA also requires an externally mounted power source or some sort of additional circuitry capable of powering the LNA. An additional DC coupler device can be employed, but this device still requires additional active electronic circuitry and a secondary connection to the power source.